Electrochemical sensors are very important candidates for many sensing and actuation applications. Micro/Nano scale techniques are very appealing in designing these sensors for completely integrated systems. Such systems can provide accurate and long-term measurements of multiple analyte for many different applications under diverse environments. As such sensors are made smaller and smaller and fully integrated, special approaches need to be taken to get enough sensitivity, stability and longevity of such sensors. Here, we provide details of CMOS compatible fabrication techniques to fabricate such sensors on very small scales using micro/nano scale fabrication and surface modification techniques. We will also present novel in situ functionalization and interface design techniques to detect different species in different environments. We also show the integration of an integrated and stable reference electrode on the same substrate using CMOS compatible fabrication techniques. Finally, we will show examples of sensing Glucose, pH, pO2 and DNA concentration in biological samples using such sensors. We’ll also show the design of the completely integrated platform and the possible applications of this technology in different areas.